1. Field of the Invention
The present invention relates generally to photolithographic illumination systems. More particularly, this invention relates to cleaning a reticle prior to its use in a photolithography tool.
2. Related Art
Photolithography (also called microlithography or optical lithography) is a semiconductor device fabrication technology. Photolithography (hereinafter referred to as lithography) uses ultraviolet or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes and transistors, can be fabricated as integrated circuits using lithographic techniques. Exposure systems or tools are used to implement lithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) containing a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist covered semiconductor wafer. The illumination system illuminates the reticle with a preferably rectangular slot illumination field to produce an image of the reticle circuit pattern. The projection system projects the image of the reticle circuit pattern onto the wafer.
The illumination system of a lithographic tool includes a light source. Excimer lasers are one such light source and operate at several characteristic wavelengths ranging from vacuum ultraviolet light to greater than 400 nanometers (nm) depending on the gas mixture used, as represented below.
By shortening the wavelength of the light, the resolution of the optical projection system is improved. Thus, in a lithography tool it is desirable to utilize a light source with wavelengths within the vacuum ultraviolet range, i.e., below 200 nm.
As shorter wavelength light sources are used in lithography, organic contamination in the exposure area of the lithography tool becomes a greater problem. It is well known that organic contaminates have high optical absorption coefficients at shorter wavelengths, particularly at 157 nm. A 1 nm film of organic contaminant belonging to the alkane group will drop the optical transmission at 157 nm by 1%. Further, an acetone residue left on the surface of a calcium fluoride optical element reduces the transmission by 4% at 157 nm. (See, T. M. Bloomstein et al., Optical Materials and Coatings at 157 nm, 3676 S.P.I.E. Proceedings 342-9 (1999) incorporated herein by reference). Optical intensity is an important issue as the number of optical elements increases in a lithography tool. It is for this reason that organic contamination can be detrimental to optical elements in 157 nm lithography.
Sources of organic contamination within a lithography tool include out-gassed products from polymer materials and solvents used for degreasing tool parts. Extremely low levels of organic contamination are critical for the exposure path in the lithography tool, and an active purge system and strict material selection are required for those areas of the tool associated with this path. The areas of the lithography tool that are controlled in this manner to keep contamination to a minimum are referred to as clean.
There are areas of a lithography tool for which cleanliness is not critical for operation such as the reticle storage area. Organic contamination is assumed to be present in such areas. However, with use of shorter wavelength lithography tools that have a greater sensitivity to organic contaminants, it is necessary to maintain even the reticle storage area in a clean state. This practice is necessary to avoid introduction of organic contaminates to the exposure system. Thus, an additional area of the lithography tool is added to the areas within which it is critical to maintain a clean environment. This increases the cost of the lithography tool and maintenance thereof Therefore, it is desirable to develop a system to avoid adding the reticle storage area to the critical list of areas of a lithography tool that must be maintained as a xe2x80x9ccleanxe2x80x9d environment.
The present invention is directed to integrating a cleaning station with a lithography tool to eliminate the need to maintain the reticle storage area (also known as the reticle library) as a clean environment. According to the present invention, the reticle would be cleaned directly prior to use. Therefore, special xe2x80x9ccleanxe2x80x9d storage is not necessary for the reticle library. The reticle cleaning procedure permits a library of reticles inside the lithography tool to be maintained without adding unnecessary environmental constraints. The reticles are retrieved from the reticle library and translated to the cleaning station in which the cleaning process occurs. Upon completion of the cleaning process, the reticle meets the cleanliness requirements for use in the exposure area of the lithography tool.
The reticle cleaning process of the present invention can be conducted at room temperature, atmospheric pressure and in an oxygen-containing environment. Moreover, the reticle cleaning process is performed in situ, thereby permitting the lithography tool to perform exposure using one reticle while the next reticle to be used is being cleaned.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.